The Effects of Na₂O and SiO₂ on Liquid Phase Sintering of Bayer Al₂O₃

Abstract

To determine how grain-boundary composition affects the liquid phase sintering of MgO-free Bayer process aluminas, samples were singly or co-doped with up to 1029 ppm Na 2 O and 603 ppm SiO 2 and heated at 1525°C up to 8 h. Na 2 O retards densification of samples from the onset of sintering and up to hold times of 30 min at 1525°C compared to the undoped samples, but similar to the as-received, MgO-free Al 2 O 3 , Na 2 Odoped samples sinter to 98% density with average grain sizes of~3 lm after 8 h. Increasing SiO 2 concentration significantly retards densification at all hold times up to 8 h. The estimated viscosities (20 -400 PaÁs) of the 0.3 to 1.8 nm thick siliceous grain-boundary films in this study indicate that diffusion greatly depends on the composition of the liquid grain-boundary phase. For low Na 2 O/SiO 2 ratios, densification of Bayer Al 2 O 3 at 1525°C is controlled by diffusion of Al 3+ through the grain-boundary liquid, whereas for high Na 2 O/SiO 2 ratios, densification can be governed by either the interface reaction (i.e., dissolution) of Al 2 O 3 or diffusion of Al 3+ . Increasing Na 2 O in SiO 2 -doped samples increases diffusion of Al 3+ and Al 2 O 3 solubility in the liquid, and thus densification increases by 1%. Based on these findings, we conclude that Bayer Al 2 O 3 densification can be manipulated by adjusting the Na 2 O to SiO 2 ratio.

Powder chemistry effects on the sintering of MgO‐doped specialty Al2O3

“…A chemically purified MgO‐doped specialty alumina powder (Almatis, Inc., Leetsdale, PA) was chosen for this study. Earlier we studied the influence of SiO 2 and Na 2 O on the sintering of MgO‐free specialty alumina, using a powder with similar physical and chemical characteristics as the powder used in this study. Since the sample preparation procedures of the two powders were identical, the differences in sintering behavior after doping with similar Na 2 O and SiO 2 concentrations should be attributable primarily to the difference in MgO concentration and its cross effects with Na 2 O and SiO 2 .…”

“…3H 2 O, ACS grade, BDH, VWR International LLC, West Chester, PA) and tetraethyl orthosilicate (Si(OC 2 H 5 ) 4 ), 98%, Aldrich Chemical Company, Inc. Milwaukee, WI), respectively, to obtain chemistries similar to commercial high purity specialty aluminas with different liquid volume fractions and different Na 2 O/SiO 2 ratios, as shown in Table . The amount of liquid in the samples at 1525°C was estimated from the Al 2 O 3 ‐Na 2 O‐SiO 2 and Al 2 O 3 ‐MgO‐SiO 2 phase diagrams based on the concentrations of SiO 2 , Na 2 O and MgO. The detailed doping procedures are described in previous work …”

“…In our previous work, the influence of Na 2 O and SiO 2 on the sintering of MgO‐free specialty alumina was investigated . Na 2 O was shown to initially retard the densification of MgO‐free specialty alumina samples, but no effect was observed after extended sintering time (>30 minutes) at 1525°C, whereas the addition of SiO 2 to MgO‐free specialty alumina formed a liquid phase and significantly retarded densification throughout the entire sintering process.…”

“…Based on phase equilibria, the solubility of Al 2 O 3 in the liquid phase was shown to increase as the Na 2 O concentration in the liquid grain boundary phase increases. Furthermore, a higher Na 2 O concentration decreases the viscosity of the liquid phase, which further increases the densification rate . Hence, samples with high Na 2 O/SiO 2 ratios (≥0.5) showed higher densities than samples with lower Na 2 O/SiO 2 ratios.…”

“…In this paper, we report how Na 2 O and SiO 2 influence the sintering of 99.8%‐99.9% pure specialty alumina doped with 380 ppm MgO. Dilatometry and sintering kinetics of MgO‐free specialty alumina samples with similar Na 2 O and SiO 2 concentrations from previous work are compared to the present results to identify the stages at which MgO affects densification, and to identify key mechanisms that are responsible for the beneficial effect of MgO on the sintering of specialty alumina. High‐resolution transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) measurements show the distribution of dopants/impurities on grain boundaries.…”